Generally, many of pigments having high tinting strength and a clear color tone have fine primary particles. However, these pigments are generally aggregates of primary particles. It is required to disperse them so as to convert their aggregate state into a finer state for bringing out the properties of the pigments themselves. However, as pigment particles become finer, generally, it becomes difficult to disperse the pigment particles homogeneously.
Dispersions in poor dispersing state have a high viscosity in many cases and it is difficult to handle them, e.g. disperse or transport them. In some cases, the dispersions undergo gelation and thus become impossible to use. Further, when such a dispersion is mixed with a different kind of pigment, a phenomenon such as segregation or settling due to the aggregation of pigments causes uneven coloring of a coated matter or a considerable decrease in tinting strength in some cases. In addition, poor dispersing causes a failure such as a decrease in the gloss of a coating film or poor leveling in some cases.
Further, some of organic pigments cause a change of a pigment crystal state in a dispersion medium in some cases. That is, it is a phenomenon where pigment particles, which are energetically unstable, change in their size or shape in a non-aqueous vehicle and their crystal state converts to a more stable state. In such a case, a failure such as a considerable decrease in tinting strength, a change in hue or the generation of coarse particles occurs generally so that the commercial value decreases considerably.
It is known that it is effective to use a variety of derivatives having a pigment structure or a structure close to it for overcoming the above problems. A variety of structures, such as a pigment derivative obtained by introducing an acidic group, a basic group or a functional group such as a phthalimide methyl group into a pigment structure or a pigment derivative obtained by bonding a pigment structure to part of a resin, have been conventionally disclosed. These structures have been long used for uses such as a dispersing agent, a particle growth inhibitor or a crystal transition inhibitor.
JP-A-2003-167112, JP-A-2002-179979, JP-2002-121418, JP-A-2002-121457 and JP-A-2001-335711 disclose compounds having a quinophthalone structure as a pigment dispersing agent. These compounds are used for a colored curable composition such as a resist ink for a color filter. Each of these compounds has the effect of improving a dispersing state but does not have sufficient properties for preparing a pigment dispersion in which pigment particles are finely dispersed and which is excellent in fluidity and storage stability with the passage of time.